1. Field of the Invention
This invention relates to a semiconductor laser element, a semiconductor laser apparatus and a method of manufacturing the apparatus, and more particularly to a multiple transverse mode high-output semiconductor laser element which is large in width of the active region or width of the light emitting portion, an apparatus using the laser element and a method of manufacturing the apparatus.
2. Description of the Related Art
Nowadays semiconductor lasers have been put into practice in various fields. Especially, wide-stripe semiconductor lasers having an oscillation wavelength in the 0.7 to 1.6 xcexcm band has come to be in wide use with increase in output power as a pumping light source for a solid sate laser, a fiber amplifier, a fiber laser and the like, as a primary light source for second harmonic generation, as a light source for forming an image by a laser-thermal system on a thermal recording medium, for instance, in printing, as a medical light source, and a light source for laser material processing and a soldering. For these applications, that the semiconductor laser is of a high output power is very important.
A multiple transverse mode high-output semiconductor laser which is not smaller than about 10 xcexcm in width of the light emitting portion and is not shorter than several thousands of hours in guaranteed life has been put into practice. For example, such a high-output semiconductor laser can operate continuously at an output of 1.5 W with a width of light emitting portion of about 50 xcexcm. For example, a semiconductor laser which comprises an InGaAsP quantum-well, an InGaP optical waveguide layer and an AlGaAs clad layer and is 50 xcexcm in stripe width and 810 nm in oscillation wavelength has been empirically proved to be sufficiently practicable at 1.5 W. In this case, high reliability at high output power is realized by virtue of a high peak light density and a light exit face temperature lowering effect obtained by increase in thickness of layers which can be realized by use of an aluminum-free active layer and an optical waveguide layer whose electric resistance is reduced by doping.
As techniques of obtaining high reliability at high output power, there have been known, for instance, a technique in which the light exit end face is specially processed or is applied with a protective layer (IEEE J. Selected Topics in Quantum Electronics, vol. 5, p. 832 (1999)) and a technique in which the absorbance index near the light exit end face is reduced (D. F. Welch, W. Streifer, R. L. Thornton, and T. Paoli: Electron. Lett. vol. 23, p. 525 (1987)).
As for a multiple transverse mode high-output semiconductor laser which is not smaller than about 50 xcexcm in stripe width, among those which make laser oscillation at 0.87 xcexcm, there have been reported a laser which is 100 xcexcm in stripe width and the catastrophic optical damage of which is 11.3 W, and a laser which is 200 xcexcm in stripe width and the catastrophic optical damage of which is 16.5 W (Electronics Letters, vol. 34, No. 2, p. 184 (1998)).
Each of the semiconductor lasers has a light emitting region (active region) which is substantially of a single layer and the light distribution in the direction normal to the active layer is confined in a micro space in the semiconductor which is as small as a half of the wavelength. Accordingly, the light density is high and since heat is generated in a narrow region, temperature elevation at the light exit end face is large, which limits increase in output power.
Methods in which a plurality of active regions are provided in a direction normal to the respective growth layers of a semiconductor laser have been proposed. In xe2x80x9cAppl. Phys. Lett. vol. 41, p. 499 (1982)xe2x80x9d, there is disclosed a method in which, in a full-face electrode type laser of a width of 100 xcexcm, three double heterostructures (DH) are superposed one on another with a P+N+-tunnel junction intervening therebetween. In this structure, the active layers are spaced from each other by at least 2 xcexcm, and the space between the active layers is larger than the wavelength, whereby the light density distribution is enlarged. However, when the growth layer side is fused to a heat sink for continuous oscillation, heat dissipation is feasible only in one direction and accordingly heat dissipation from the three active layers each forming a heat generating region is limited, which results in larger temperature elevation in active layers remote from the heat sink and in deterioration in reliability.
In the method disclosed in xe2x80x9cAppl. Phys. Lett. vol. 42, p. 850 (1983)xe2x80x9d, output power of a laser is increased by providing a plurality of active layers in an optical waveguide region which is thickened to at least 2 xcexcm. In this case, though peak light intensity can be lowered, the structure merely results in a pulse-driven full-face electrode laser of a width of 250 xcexcm and hardly contributes to suppressing temperature elevation at the light exit end face during continuous oscillation or heat dissipation. Further, in Japanese Unexamined Patent Publication No. 4(1992)-157777, there is disclosed a semiconductor laser which is arranged to pump a solid state laser at a higher output power by superposing a pair of wide stripe chips, with an electrode intervening therebetween, provided with a stripe-like light radiating portion at the center thereof. However, this arrangement is disadvantageous in that since a pair of PN-junctions are superposed to form a PNPN-junction, it is difficult to uniformly excite the two laser chips in a controlled manner.
In view of the foregoing observations and description, the primary object of the present invention is to provide a high-output semiconductor laser element which is highly reliable and higher in maximum optical power.
Another object of the present invention is to provide a high-output semiconductor laser apparatus which is highly reliable and is increased in maximum optical output.
Still another object of the present invention is to provide a method of easily manufacturing such a high-output semiconductor laser apparatus.
In accordance with a first aspect of the present invention, there is provided a high-output semiconductor laser element comprising a plurality of laser structures, each comprising at least one active layer interposed between a P-type clad layer and a N-type clad layer, which are superposed on a substrate one on another with a P+N+-tunnel junction intervening between each pair of the laser structures, the active region of each of the laser structures being not smaller than 10 xcexcm and not larger than 80 xcexcm in width, the distance h between the active layers which are most distant from each other in the active layers of the laser structures being not larger than the width W of the active region which is the widest in the laser structures, and the width of said semiconductor laser element being not smaller than W+2h.
It is preferred that heat sinks be provided on both sides of the semiconductor laser element, the laser structure side and the substrate side.
When heat sinks are provided, the thickness of the semiconductor laser element is preferably not larger than 100 xcexcm and more preferably not larger than 80 xcexcm.
Though the high-output semiconductor laser element in accordance with the first aspect of the present invention has a plurality of laser structures or active regions, the peak light intensity at the light exit end face during high power operation can be reduced since the active regions are separated from each other, whereby deterioration of the light exit end face due to photochemical reaction and the like can be suppressed. Further, by limiting the width of the active region of each laser structure to 80 xcexcm, the heat flow parallel to the active layers can be effectively used or the non-light emitting portion can be effectively used as a heat dissipation path, whereby temperature elevation at the end face can be suppressed and deterioration of the laser element during continuous operation can be suppressed.
Further, since the distance h between the active layers which are most distant from each other in the active layers of the laser structures is not larger than the width W of the active region which is the widest in the laser structures (hxe2x89xa6W), the virtual light emitting region, or the light spot, is within the stripe width in both the longitudinal size and the lateral size, whereby the laser element of the first aspect practically arises no problem.
Further, by providing an additional heat sink on the substrate side in addition to the normal heat sink on the epitaxial growth layer side, the heat dissipation effect can be further improved and the temperature elevation during continuous oscillation can be suppressed, whereby reliability at high output power can be improved.
The high-output semiconductor laser element having an active region which is 10 to 80 xcexcm in width is useful as a light source for thermal recording mode in printing to increase the write speed and to lower the sensitivity of the recording medium. Further, when the laser element of the first aspect of the present invention is continuously operated to pump a solid state laser or a fiber laser, higher power pumping light can be output for a given area, the output power of the solid state laser or the fiber laser can be easily increased. For the laser processing field, the medical field and the like, a laser element of high reliability at high output power highly contributes to improving the reliability of the system.
The solid state laser or fiber laser pumping effect can be increased by use of a semiconductor laser which can emit a laser beam in a relatively small beam spot size, about several tens xcexcm. However, laser beams from conventional semiconductor lasers are generally 1 xcexcm at most in vertical width though can be about several tens xcexcm in horizontal width and highly asymmetric in shape. As a pumping laser, in crease in vertical beam diameter within the largest width gives rise to no problem. Further, for such application, it is important to continuously operate at high output power, deterioration of the end face of the element can be suppressed by better use of the horizontal heat dissipation effect when the width of the active region is as small as 10 to 80 xcexcm. Thermal influence when the laser is uncontinuously operated is substantially the same as when the laser is continuously operated except unusual cases where the laser is driven by pulses which is relatively small in duty ratio. For example, the laser is modulated, for instance, in CTP (Computer to Plate) in thermal mode, and pulses of a duty ratio up to several tens % are used in order to generate heat more effectively. In such a case, thermal influence is substantially the same as when the laser is continuously operated. The semiconductor laser element of the first aspect is also useful for such application since it can form an image with a spot diameter of 30 to 80 xcexcm. The laser element of the first aspect is especially useful when the duty ratio is not smaller than 1% or when the pulse width is not smaller than 1 xcexcs where influence of heat is serious.
In accordance with a second aspect of the present invention, there is provided a high-output semiconductor laser element comprising a plurality of laser structures, each comprising at least one active layer interposed between a P-type clad layer and a N-type clad layer, which are superposed on a substrate one on another with a P+N+-tunnel junction intervening between each pair of the laser structures, the active region of each of the laser structures being not smaller than 10 xcexcm and not larger than 100 xcexcm in width, the distance h between the active layers which are most distant from each other in the active layers of the laser structures being not larger than the virtual width W of the active region of the laser element formed by the active regions of the laser structures, the width of said semiconductor laser element being not smaller than W+2h, and each of the laser structures being provided with at least one of a current-blocking structure and a graded index optical waveguide structure.
The active regions of the laser structures may be in the same position in the direction of width of the laser element or may be in different positions in the direction of width of the laser element within a range of not larger than 100 xcexcm.
The xe2x80x9cvirtual width W of the active region the laser elementxe2x80x9d is the width of the active region which is the widest in the laser structures when the active regions of the laser structures are in the same position in the direction of width of the laser element, and is the distance in the direction of width of the laser element between the edges of the active regions which are most distant from each other in the direction of width of the laser element.
It is preferred that the laser structure which is most distant from the substrate in the laser structures be of a ridge optical waveguide type.
The laser structures except the laser structure which is most distant from the substrate in the laser structures may have a plurality of stripe-like active regions.
It is preferred that heat sinks be provided on both sides of the semiconductor laser element, the laser structure side and the substrate side.
When heat sinks are provided, the thickness of the semiconductor laser element is preferably not larger than 100 xcexcm and more preferably not larger than 80 xcexcm.
Though the high-output semiconductor laser element in accordance with the second aspect of the present invention has a plurality of laser structures or active regions, the peak light intensity at the light exit end face during high power operation can be reduced since the active regions are separated from each other, whereby deterioration of the light exit end face due to photochemical reaction and the like can be suppressed. Further, by limiting the width of the active region of each laser structure to 100 xcexcm, the heat flow parallel to the active layers can be effectively used or the non-light emitting portion can be effectively used as a heat dissipation path, whereby temperature elevation at the end face can be suppressed and deterioration of the laser element during continuous operation can be suppressed. This effect is further enhanced when the width of the active region of each laser structure is not larger than 80 xcexcm.
Since each of the laser structures is provided with at least one of a current-blocking structure and a graded index optical waveguide structure, the laser element of the second aspect can produce a high quality laser beam without a kink in the optical output versus current characteristics.
Further, since the distance h between the active layers which are most distant from each other in the active layers of the laser structures is not larger than the virtual width W of the active region of the laser element formed by the active regions of the laser structures (hxe2x89xa6W), the virtual light emitting region, or the light spot, is within a predetermined range in both the longitudinal size and the lateral size, whereby the laser element of the second aspect practically arises no problem.
When the laser structures except the laser structure which is most distant from the substrate in the laser structures have a plurality of stripe-like active regions, the heat flow parallel to the active layers can be effectively used.
Further, by providing an additional heat sink on the substrate side in addition to the normal heat sink on the epitaxial growth layer side, the heat dissipation effect can be further improved and the temperature elevation during continuous oscillation can be suppressed, whereby reliability at high output power can be improved.
The high-output semiconductor laser element having an active region which is 10 to 100 xcexcm in width is useful as a light source for thermal recording mode or as a light source for forming a high quality image in printing to increase the write speed and to lower the sensitivity of the recording medium. Further, when the laser element of the second aspect of the present invention is continuously operated to pump a solid state laser or a fiber laser, it can be a high quality pumping light source free from fluctuation in output power or less in noise. Further, since the laser element of the second aspect can output higher power pumping light for a given area, the output power of the solid state laser or the fiber laser can be easily increased. For the laser processing field, the medical field and the like, a laser element of high reliability at high output power highly contributes to improving the reliability of the system.
The solid state laser or fiber laser pumping effect can be increased by use of a semiconductor laser which can emit a laser beam in a relatively small beam spot size, about several tens xcexcm in diameter. However, laser beams from conventional semiconductor lasers are generally 1 xcexcm at most in vertical width though can be about several tens xcexcm in horizontal width and highly asymmetric in shape. As a pumping laser, increase in vertical beam diameter within the largest width gives rise to no problem. Further, for such application, it is important to continuously operate at high output power, deterioration of the end face of the element can be suppressed by better use of the horizontal heat dissipation effect when the width of the active region is as small as 10 to 100 xcexcm (or 10 to 80 xcexcm). Thermal influence when the laser is uncontinuously operated is substantially the same as when the laser is continuously operated except unusual cases where the laser is driven by pulses which is relatively small in duty ratio. For example, the laser is modulated, for instance, in CTP (Computer to Plate) in thermal mode, and pulses of a duty ratio up to several tens % are used in order to generate heat more effectively. In such a case, thermal influence is substantially the same as when the laser is continuously operated. The semiconductor laser element of the second aspect is also useful for such application since it can form an image with a spot diameter of 30 to 80 xcexcm. The laser element of the second aspect is especially useful when the duty ratio is not smaller than 1% or when the pulse width is not smaller than 1 xcexcs where influence of heat is serious.
In accordance with a third aspect of the present invention, there is provided a high-output semiconductor laser apparatus comprising
a semiconductor laser element including a plurality of laser structures which are superposed one on another with a P+N+-tunnel junction intervening between each pair of the laser structures, each of the laser structures comprising at least one active layer interposed between a P-type clad layer and a N-type clad layer and at least two of the laser structures being spaced from each other by a substrate, and
a pair of heat sinks which are respectively provided on the upper and lower surface of the semiconductor laser element,
the active region of each of the laser structures being not smaller than 30 xcexcm and not larger than 500 xcexcm in width, and
the distance between the active layers which are most distant from each other in the active layers of the laser structures being not larger than the virtual width of the active region of the laser element formed by the active regions of the laser structures.
The active regions of the laser structures may be in the same position in the direction of width of the laser element or may be in different positions in the direction of width of the laser element within a range of not larger than 500 xcexcm.
The xe2x80x9cvirtual width of the active region the laser elementxe2x80x9d is the width of the active region which is the widest in the laser structures when the active regions of the laser structures are in the same position in the direction of width of the laser element, and is the distance in the direction of width of the laser element between the edges of the active regions which are most distant from each other in the direction of width of the laser element.
It is preferred that each of the laser structures be provided with at least one of a current-blocking structure and a graded index optical waveguide structure.
In accordance with a fourth aspect of the present invention, there is provided a method of manufacturing the laser apparatus of the third aspect of the present invention comprising the steps of
forming a semiconductor laser element by separately forming a plurality of laser structures, each comprising a P-type clad layer, a N-type clad layer and at least one active layer interposed therebetween which are superposed on a substrate, each of the active regions being not smaller than 30 xcexcm and not larger than 500 xcexcm in width, forming a p+-semiconductor layer and an n+-semiconductor layer on the uppermost layer of predetermined one of the laser structures and bonding the side of the superposed layers of the predetermined laser structure to the lower surface of the substrate of one of the other laser structures, and
providing a pair of heat sinks respectively on the upper and lower surfaces of the laser element.
Though the high-output semiconductor laser apparatus in accordance with the third aspect of the present invention has a plurality of laser structures or active regions, the peak light intensity at the light exit end face during high power operation can be reduced since the active regions are separated from each other, whereby deterioration of the light exit end face due to photochemical reaction and the like can be suppressed.
Since each of the laser structures is provided with at least one of a current-blocking structure and a graded index optical waveguide structure, the laser apparatus of the third aspect can produce a high quality laser beam without a kink in the optical output versus current characteristics.
By inserting a substrate between the active layers, the distance between the active layers can be easily enlarged, which contributes to reduction of light density and enhancement in heat dissipation effect.
Further, since the distance between the active layers which are most distant from each other in the active layers of the laser structures is not larger than the virtual width of the active region of the laser element formed by the active regions of the laser structures, the virtual light emitting region, or the light spot, is within a predetermined range in both the longitudinal size and the lateral size, whereby the laser apparatus of the third aspect practically arises no problem.
Further, since heat sinks are provided both on the upper and lower sides of the laser element, the heat dissipation effect can be further improved and the temperature elevation during continuous oscillation can be suppressed, whereby reliability at high output power can be improved.
In accordance with the method of manufacturing a laser apparatus, a laser apparatus can be manufactured in a shorter time as compared with when the layers are formed in sequence by crystal growth or the like. Further, by inserting a substrate between the active layers, the distance between the active layers can be easily enlarged, a laser apparatus can be manufactured in a shorter time as compared with when the layers are grown to enlarge the distance between the active layers.